The present invention relates to a fluid-filled bushing for supporting an end portion of a suspension arm on a vehicle body, the suspension arm suspending a wheel of a motor vehicle.
The characteristics of a conventional fluid-filled bushing for supporting a suspension arm of a motor vehicle are tuned up for the following purposes:
(1) to reduce longitudinal resonance below a spring of a suspension caused by increase of damping force (that is, loss coefficient tan .delta.) in the vicinity of 18 Hz as an input frequency (see FIG. 9); PA1 (2) to reduce air-columnar resonance sound of a tire caused by reduction of a dynamic spring constant K* in the vicinity of 230 Hz as an input frequency (see FIG. 10); and PA1 (3) to reduce cavity resonance sound (drumming) peculiar to a cabin due to reduction of the dynamic spring constant K* in the vicinity of 80 Hz as an input frequency (see FIG. 10). As a result, comfortableness to drive or road noise is improved in the input frequency ranged up to the vicinity of 230 Hz.
However, in the above-mentioned conventional bushing, it was impossible to sufficiently improve road noise in a high-frequency range of not less than 500 Hz which is a rigid resonance frequency range of a suspension arm. There was a problem that rustling noise is transmitted from the road surface.
To improve road noise in the above-mentioned high-frequency range, the present applicant made up an anisotropic bushing in which a hollow (space) was formed inside a bushing rubber of a solid bushing having no fluid chamber, and tested a real vehicle mounted with the anisotropic bushing for road noise. FIG. 11 shows the anisotropic bushing. In a bushing 02 disposed in an end portion of a suspension arm 01 to be attached to a vehicle body, a pair of upper and lower hollows 04 and 04 are formed in a bushing rubber 03 of the bushing 02. By means of the hollows 04 and 04 formed in the bushing rubber 03 in the above manner, a dynamic spring constant K* in the up/down direction (Z-Z' direction) having a large connection with road noise can be reduced to about 1/3 in a broad frequency range, while a static spring constant in the vehicle left/right direction (X-X' direction) having a large connection with steering stability performance of the vehicle is ensured sufficiently (see FIG. 12).
Although the reduction of road noise was expected in a high frequency range by the reduction of the above-mentioned dynamic spring constant K*, the test of the real vehicle mounted with the bushing resulted in, far from reducing, increasing road noise in the high frequency range. It therefore became clear that road noise in a high frequency range could not be improved only by reducing a dynamic spring constant K* of a bushing. The reason will be described below.
FIG. 13 shows a model of suspension of a motor vehicle, in which an equivalent mass of a suspension arm and a spring of a bushing are connected in series between a vehicle body and a vibration input point (a connection portion with a knuckle). A transmission force N which is a parameter expressing the size of road noise is defined as a maximum value of a force transmitted from the suspension arm to the vehicle body. A resonance magnification .upsilon. is defined as (maximum value of output amplitude x)/(input amplitude x.sub.0). Further, the above-mentioned rigid resonance frequency f.sub.0 is defined as a frequency with which the suspension arm resonates. The transmission force N is given by the resonance magnification U and the dynamic spring constant K* in the following relation: EQU transmission force N=resonance magnification .upsilon..times.dynamic spring constant K*
As is apparent from the above expression, if only the dynamic spring constant K* is reduced, the transmission force N is not always reduced when the resonance magnification .upsilon. is large. It is therefore impossible to improve high-frequency road noise in a range of the rigid resonance frequency f.sub.0.